The invention relates to a circuit arrangement to generate square-wave signals in which the frequency of recurrence of the output square-wave signals coincides with the frequency of input signals and in which the square-wave signals have a duty cycle.
It is already known generally to generate square-wave signals which have the same recurrence frequency as input signals and a specified duty cycle. Such a circuit arrangement is disclosed, for example, in British Patent Application No. 2,019,685, which shows two capacitors coupled via a diode. The respective charging and discharging time constants of these capacitors are utilized to divide the time interval between two successive input pulses into two time periods of a given ratio of their duration. The first capacitor is discharged across a transistor switch directly controlled by the input signal. Therefore, the progress of the discharge process depends not only on the unavoidable discharge time constant, but also upon the length of the input signal. Furthermore, the time when the second capacitor turns from the discharge state to the charging state is determined by the voltage at the first capacitor during its subsequent charging as a function of the threshold voltage of the diode. This threshold voltage is, therefore, a direct tolerance factor of the circuit. Finally, although known circuit is adapted to follow the recurrence frequency variations of the input signal, it fails to control brief variations of the recurrence frequency (jitter).
In addition, it is also known to generate such square-wave signals by means of circuit arrangements having a monostable flip-flop as the input stage. This flip-flop is set by the input signals and resumes its reset position after a constant period of time. if the frequency of the input signals changes, the recurrence frequency of the square-wave signals also changes accordingly. However, since the time during which the monostable flip-flop is set is normally constant, the duty cycle will change if the frequency of the input signals changes.
A further development of this general circuit concept is disclosed in French Patent Application No. 2,425,178 which relates to a signal generator for an electronic ignition system, which is used to set the firing time for an internal combustion engine. Here, too, the problem arises, even though in connection with a totally different application, to provide, at a variable input signal frequency, an output signal of the same recurrence frequency and specified duty cycle. The flip-flop provided is set by the input signals, and by means of its output signals a capacitor is discharged at a specified first time constant until the discharge reaches a given threshold value. At this time a comparator starts to conduct and emits a reset signal for the flip-flop. The capacitor is then recharged with a given second time constant until the flip-flop is set again by the next input pulse.
At first glance, this circuit arrangement seems to meet all requirements for the present invention and, in the case of its application in electronic ignition systems, does work satisfactorily. But marginal conditions are present here which do not always prevail; it may be assumed that starting an engine always begins at a recurrence frequency near zero, the capacitor then also being charged to its maximum charge and that, overall, the recurrence frequency is relatively low (i.e. ranging up to several kHz). As the recurrence frequency of the input signals increases (i.e. with increasing engine speed) the output signal frequency then increases accordingly at approximately the same duty cycle.
But such favorable operating conditions do not often prevail (e.g. in servo clock motors for magnetic disc memories). In the application mentioned it must be assumed that the input signals are of much higher recurrence frequency, (in the MHz range) and--being of particularly aggravating effect--that relative to the automobile above application the operating condition changes suddenly. This means that as soon as input signals appear at all, they appear immediately at a relatively higher recurrence frequency which, however, also changes slowly--again in relation to the above application. In such an application the known circuit would not find sufficient time to center with respect to its fixed lower threshold value. Critical in the known circuit, therefore, is the initial state which restricts its application to more continually progressing control processes.
Also known generally are synchronizing circuits utilizing, for instance commercially available integrated circuits known as "phase locked loops". These synchronizing circuits generate square-wave signals coinciding with the input signals as to phase and frequency. But they are relatively expensive because they contain, for instance, an oscillator which generates the square-wave signals and whose recurrence frequency is varied as a function of the phase difference between the square-wave signals and the input signals.